This invention relates to flat-panel displays of the cathode-ray tube (xe2x80x9cCRTxe2x80x9d) type, including the fabrication of flat-panel CRT displays.
A flat-panel CRT display basically consists of an electron-emitting component and a light-emitting component. The electron-emitting component, commonly referred to as a cathode, contains electron-emissive regions that emit electrons over a relatively wide area. The emitted electrons are suitably directed towards light-emissive elements distributed over a corresponding area in the light-emitting component. Upon being struck by the electrons, the light-emissive elements emit light that produces an image on the display""s viewing surface.
The electron-emitting and light-emitting components are connected together to form a sealed enclosure normally maintained at a pressure much less than 1 atm. The exterior-to-interior pressure differential across the display is typically in the vicinity of 1 atm. In a flat-panel CRT display of significant viewing area, e.g., at least 10 cm2, the electron-emitting and light-emitting components are normally incapable of resisting the exterior-to-interior pressure differential on their own. Accordingly, a spacer (or support) system is conventionally provided inside the sealed enclosure to prevent air pressure and other external forces from collapsing the display.
The spacer system typically consists of a group of laterally separated spacers positioned so as to not be directly visible on the viewing surface. The presence of the spacer system can adversely affect the flow of electrons through the display. For example, electrons coming from various sources occasionally strike the spacer system, causing it to become electrically charged. The electric potential field in the vicinity of the spacer system changes. The trajectories of electrons emitted by the electron-emitting device are thereby affected, often leading to degradation in the image produced on the viewing surface.
More particularly, electrons that strike a body, such as a spacer system in a flat-panel display, are conventionally referred to as primary electrons. When the body is struck by primary electrons of high energy, e.g., greater than 100 eV, the body normally emits secondary electrons of relatively low energy. More than one secondary electron is, on the average, typically emitted by the body in response to each high-energy primary electron striking the body. Although electrons are often supplied to the body from one or more other sources, the fact that the number of outgoing (secondary) electrons exceeds the number of incoming (primary) electrons commonly results in a net positive charge building up on the body.
It is desirable to reduce the amount of positive charge buildup on a spacer system in a flat-panel CRT display. Jin et al, U.S. Pat. No. 5,598,056, describes one technique for doing so. In Jin et al, each spacer in the display""s spacer system is a pillar consisting of multiple layers that extend laterally relative to the electron-emitting and light-emitting components. The layers in each spacer pillar alternate between an electrically insulating layer and an electrically conductive layer. The insulating layers are recessed with respect to the conductive layers so as to form grooves. When secondary electrons are emitted by the spacers in Jin et al, the grooves trap some of the secondary electrons and prevent them from escaping the spacers. Because fewer secondary electrons escape the spacers than what would occur if the grooves were absent, the amount of positive charge buildup on the spacers is reduced.
The technique employed in Jin et al to reduce positive charge buildup is creative. However, the spacers in Jin et al are relatively complex and pose significant concerns in dimensional tolerance and, therefore, in reliability. Manufacturing the spacers in Jin et al could be problemsome. It is desirable to have a relatively simple technique, including a simple spacer design, for reducing charge buildup on a spacer system of a flat-panel CRT display.
The present invention furnishes a flat-panel display in which a spacer situated between a pair of plate structures has a rough face. An image is supplied by one of the plate structures in response to electrons provided from the other plate structure. Somewhat similar to what occurs in Jin et al, the roughness in the face of the present spacer prevents some secondary electrons emitted by the spacer from escaping the spacer. Accordingly, positive charge buildup on the spacer is normally reduced. The image is thereby improved.
In particular, secondary electrons emitted by the present spacer as a result of being struck by primary electrons are, on the average, normally of significantly lower energy than the primary electrons. Due to the roughness in the spacer""s face, the lower-energy secondary electrons are more prone to impact the spacer and be captured by it than what would occur if the spacer""s face were smooth. The lower-energy secondary electrons captured by the spacer cause relatively little further secondary electron emission from the spacer.
Roughness in the face of a body being struck by primary electrons may sometimes itself cause the body to emit an increased number of secondary electrons, especially when the energy of the primary electrons is quite high. This increase in the secondary electron emission is offset by the number of secondary electrons captured by the body due to its facial roughness. In the present flat-panel display, the primary electron energy, while high, is normally sufficiently low that the roughness in the spacer""s face leads to a reduction in the overall number of secondary electrons that escape the spacer.
To the extent that the spacer used in the present flat-panel display has multiple levels of spacer material, the levels typically extend vertically relative to the electron-emitting and light-emitting components rather than laterally as in Jin et al. A spacer with vertically extending spacer-material levels is generally simpler in design, and can be fabricated to high tolerances more easily, than a spacer having laterally extending spacer-material levels. When the present spacer has multiple vertically extending levels of spacer material, reliability concerns associated with the spacer design are considerably less severe than those that arise with the spacer design of Jin et al. When the spacer used in the present display has only a single level of spacer material, the display essentially avoids the reliability concerns that arise in Jin et al. The net result is a substantial improvement over Jin et al.
A flat-panel display that employs the teachings of the invention is generally configured in the following way. The display contains a first plate structure, a second plate structure situated opposite the first plate structure, and a spacer situated between the plate structures. The first plate structure emits electrons. The second plate structure emits light to produce an image upon receiving electrons emitted by the first plate structure. The spacer has a rough face that extends at least partway from either plate structure to the other plate structure.
Primary electrons which strike the spacer include electrons that follow trajectories directly from the first plate structure to the spacer as well as electrons that reflect off the second plate structure after having traveled from the first plate structure to the second plate structure. The reflected electrons are generally referred to as xe2x80x9cbackscatteredxe2x80x9d electrons. While the flat-panel display can normally be controlled so that only a small fraction of the electrons emitted by the first plate structure directly strike the spacer, the backscattered electrons travel in a broad distribution of directions as they leave the second plate structure. As a result, electron backscattering off the second plate structure is difficult to control direction-wise.
By inhibiting secondary electrons emitted by the present spacer from escaping the spacer, the roughness in the spacer""s face also reduces spacer charging that would otherwise result from backscattered primary electrons striking the spacer. In certain embodiments of the present display, the spacer facial roughness is provided with a directional roughness characteristic that enhances the ability of the spacer""s rough face to prevent secondary electrons, especially those caused by backscattered primary electrons, from escaping the spacer.
In one aspect of the invention, the present spacer is implemented as a spacer wall. The roughness in the face of the spacer wall is adjustable according to the average strength EAV of the electric field directed from the second plate structure to the first plate structure during operation of the display. The roughness in the wall""s face can take various forms such as depressions or/and protuberances. The depressions can, for example, be implemented as pores, trenches, or/and notches. The depressions can be rounded three-dimensionally, typically to have portions of roughly constant radius of curvature. When the wall""s face is defined by grains, the depressions can be formed by valleys between adjoining grains. The protuberances can take the form of ridges, particles, pillars, or/and spires.
Regardless of the actual form of the roughness in the face of the spacer wall, the facial roughness can be approximated by identical cylindrical pores of pore diameter dp. The representation of the wall""s facial roughness using identical pores ideally has the same total electron yield coefficient that occurs with the actual roughness in the wall""s face. In this representation, the wall""s facial roughness corresponds to a wall porosity of at least 10% along the wall""s face and a pore height hp of at least 15% of pore height parameter hMD. Parameter hMD is given by the following relationship as a function of average electric field strength EAV and pore diameter dp:
hMD={square root over (2dpxcex52DMD/eEAV)}
where e is the electron charge, and xcex52DMD is the median energy of secondary electrons as they depart from (leave) the spacer wall. By using this relationship, the characteristics of the identical pores that approximate the actual roughness in the wall""s face can be suitably adjusted, as electric field strength EAV changes, to reduce the number of secondary electrons that escape the spacer wall.
Magnetic material may be present in the spacer wall along its face. The magnetic material causes the trajectories of secondary electrons emitted by the wall to be altered in a way that further inhibits them from escaping the wall.
In another aspect of the invention, the spacer contains a main spacer body having a rough face. The main body of the spacer is typically shaped like a wall but can have other shapes. The roughness in the main body""s face is achieved with pores that extend into the main body along its face. The pores have an average diameter of 1-1,000 nm and provide a porosity of at least 10% along the main body""s face.
The main body of the spacer in this aspect of the invention can be internally configured in various ways. As one example, the main body can be implemented simply as a porous electrically non-conductive substrate. The term xe2x80x9celectrically non-conductivexe2x80x9d here generally means electrically insulating or electrically resistive. A coating may overlie the substrate in a generally conformal manner. With the pores acting to inhibit secondary electrons from escaping the main body, the coating preferably contains material, such as carbon, that itself emits a relatively low level of secondary electrons.
As another example, the main body of the spacer can be implemented with a substrate and a porous layer that overlies the substrate. The porous layer normally has an average electrical resistivity of 108-1014 ohm-cm, preferably 109-1013 ohm-cm, at 25xc2x0 C. The porous layer is preferably of at least ten times greater resistance per unit length than the substrate. By implementing the main body in this way, the substrate largely determines the non-emissive electrical characteristics of the main body, while the pores largely determine the secondary electron escape characteristics of the main body. Separating these two types of spacer characteristics in this way makes it easier to design the spacer. A generally conformal coating, which typically emits a relatively low level of secondary electrons, may overlie the porous layer.
Various techniques are suitable for manufacturing the present flat-panel display, especially the spacer, in accordance with the invention. For instance, the spacer can be fabricated by a procedure that entails furnishing a composite in which support material and further material are interspersed with each other. At least of part of the further material is removed from the composite to convert it into a porous body. Depending on whether the porous body is larger than, or of approximately the same size as, the main body of the spacer, part or all of the porous body is utilized as at least part of the spacer.
The support material of the composite may be ceramic, while the further material is organic material consisting of carbon and non-carbon material. The porous body is created by removing at least part of the non-carbon material from the composite. Alternatively, the composite can be a gel or open network of solid material, while the further material is liquid. The porous body is then created by removing at least part of the liquid without causing the support material to completely fill the space previously occupied by the removed liquid.
The composite can be created according to a process in which a liquidous body is formed from a composition of the support material, the further material, and liquid. In the liquidous body, the further material may be in the form of discrete particles, typically roughly spherical in shape. The liquid is removed to transform the liquidous body into a solid composite. Alternatively, a layer of discrete particles can be formed, after which the support material is introduced into spaces between the particles. A layer of support material may also be provided below the particle layer. In either case, at least a portion of the particles are later removed from the solid composite to form the porous body.
In another technique for fabricating the present flat-panel display, an initial face of a primary body is roughened to form a rough face. The primary body may, or may not, be porous (or otherwise facially roughened) prior to the roughening step. The roughening step typically entails etching the primary body. The etching step can be performed in such a way as to impose the above-mentioned directional roughness characteristic on the primary body""s face, especially when the initial face of the primary body is defined by grains.
Alternatively, protuberances can be provided over a primary body to furnish the body with a rough face. Regardless of which of the preceding techniques is employed, part or all of the primary body forms at least part of the spacer.
When carbon is employed in the conformal coating that emits secondary electrons at a relatively low level, the carbon can be provided by chemical vapor deposition. The carbon can also be provided by thermally decomposing carbon-containing material over an underlying body that forms at least part of the spacer. This can be done subsequent to forming the underlying body or during an anneal operation used in creating the underlying body.
In short, the rough-faced spacer utilized in the present flat-panel display typically reduces the number of secondary electrons that escape the spacer, thereby reducing positive charge buildup on the spacer. The present spacer is of relatively simple configuration and can be manufactured according to readily controllable manufacturing techniques. The invention thus provides a large advance over the prior art.